Multilayer thin-film back contact system for flexible photovoltaic devices on polymer substrates

ABSTRACT

A polymer substrate and back contact structure for a photovoltaic element, and a photovoltaic element include a CIGS photovoltaic structure, a polymer substrate having a device side at which the photovoltaic element can be located and a back side opposite the device side. A layer of dielectric is formed at the back side of the polymer substrate. A metal structure is formed at the device side of the polymer substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. non-provisionalapplication Ser. No. 14/198,209 filed Mar. 5, 2014, which is acontinuation of U.S. non-provisional application Ser. No. 13/572,387filed Aug. 10, 2012, which claims the benefit of priority to U.S.Provisional Patent Application No. 61/522,209 filed Aug. 10, 2011. Eachof the above-mentioned applications is incorporated herein by reference.

BACKGROUND

1. Technical Field

This disclosure relates to photovoltaic modules and methods ofmanufacturing photovoltaic modules and, more particularly, tophotovoltaic modules and methods of manufacturing photovoltaic modulesin which mechanical distortion in the modules is substantially reducedor eliminated.

2. Discussion of the Related Art

One type of flexible photovoltaic (PV) module is formed as a thin-filmdevice on a polymeric substrate. An example of such devices is theCopper-Indium-Gallium-Selenide (CIGS) device. CIGS devices present manychallenges in terms of the thin-film deposition processes, devicepatterning, and final assembly/packaging. Polymer substrates are ofgreat significance since high-temperature variations of the material areadequate to accommodate CIGS processing while the material maintains itsdielectric properties, which enables monolithic integration without anyadditional insulating films.

A fundamental challenge in flexible CIGS devices is in the deposition ofa metallic back contact onto the polymer prior to the deposition of theCIGS p-type absorber layer. This back contact makes ohmic contact to theCIGS and allows for current to flow through the device and be collectedthrough interconnects to the leads attached to the electrical load.Thus, this back contact, which is usually a metal, must maintain highelectrical conductivity, both before and after device processing. Itmust also survive the deposition environment for the subsequent thinfilm deposition steps.

SUMMARY

According to a first aspect, a polymer substrate and back contactstructure for a photovoltaic element is provided. The structure includesa polymer substrate having a device side at which the photovoltaicelement can be located and a back side opposite the device side. A layerof dielectric is formed at the back side of the polymer substrate. Ametal structure is formed at the device side of the polymer substrate.

According to another aspect, a photovoltaic element is provided. Thephotovoltaic element includes a CIGS photovoltaic structure and apolymer substrate having a device side at which the CIGS photovoltaicstructure can be located and a back side opposite the device side. Alayer of dielectric is formed at the back side of the polymer substrate.A metal structure is formed at the device side of the polymer substratebetween the polymer substrate and the CIGS photovoltaic structure.

According to another aspect, a method for forming a photovoltaic elementincludes the following steps: (1) disposing a first adhesion layer on aback side of a polymer substrate; (2) disposing a dielectric layer onthe first adhesion layer; (3) after the step of disposing the dielectriclayer, disposing a metal structure on a device side of the polymersubstrate, the device side being opposite of the back side; and (4)disposing a CIGS photovoltaic structure on the metal structure.

According to another aspect, a method for forming a photovoltaic elementincludes the following steps: (1) disposing a dielectric layer on a backside of a polymer substrate; (2) disposing a metallic film layer on adevice side of the polymer substrate, the device side being opposite ofthe back side; (3) disposing a molybdenum cap layer on the metallic filmlayer at least partially using a vacuum-based sputter deposition processat a pressure of less than 20 millitorr; and (4) disposing a CIGSphotovoltaic structure on the molybdenum cap layer.

According to another aspect, a method for forming a photovoltaic elementincludes the following steps: (1) disposing a backside metal layer on aback side of a polymer substrate using a vacuum-based sputter depositionprocess at a pressure of less than 6 millitorr; (2) disposing a metallicfilm layer on a device side of the polymer substrate, the device sidebeing opposite of the back side; (3) disposing a molybdenum cap layer onthe metallic film layer; and (4) disposing a CIGS photovoltaic structureon the molybdenum cap layer.

According to another aspect, a photovoltaic element includes a polymersubstrate having a device side and a back side opposite the device side.A dielectric layer is disposed on the back side of the polymersubstrate, and a metallic film layer is disposed on the device side ofthe polymer substrate. A molybdenum cap layer is disposed on themetallic film layer, and the molybdenum cap layer has a density of atleast 85% of the bulk density of molybdenum. A CIGS photovoltaicstructure is disposed on the molybdenum cap layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages will be apparent fromthe more particular description of preferred aspects, as illustrated inthe accompanying drawings, in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale. In the drawings, the thickness of layers andregions may be exaggerated for clarity.

FIG. 1 includes a graph of intrinsic stress in Mo as a function of Arpressure during a vacuum-based sputtering Mo deposition process.

FIG. 2 includes a schematic cross-sectional view of a back contact for aflexible monolithically integrated CIGS photovoltaic device on a polymerutilizing a metallic multilayer as a top contact and an oxide as aback-side coating, according to some exemplary embodiments.

FIG. 3 includes an image of a dielectric-polymer-metal-Mo-CIGS stackstructure, according to some exemplary embodiments.

FIG. 4 includes a schematic cross-sectional view of a device including abilayer Mo cap layer, according to some exemplary embodiments.

FIG. 5 illustrates a method for forming a photovoltaic element,according to some exemplary embodiments.

FIG. 6 illustrates another method for forming a photovoltaic element,according to some exemplary embodiments.

FIG. 7 includes a schematic cross-sectional view of a device including abackside Mo layer, according to some exemplary embodiments.

DETAILED DESCRIPTION

For CIGS devices, molybdenum (Mo) has been a common choice of materialfor a back contact, regardless of the substrate. While Mo can bedeposited in a straightforward manner using DC sputtering or other thinfilm deposition methods, the wide range of stress states possible withsputtering can particularly complicate deposition onto flexiblesubstrates, particularly those that do not exhibit significantstiffness, such as polymers. Unlike rigid substrates where the filmstresses can readily be borne by the substrate, film stresses can have asignificant impact upon the life, surface topology, and physicalproperties of flexible substrates, particularly substrates made frompolymers. This class of substrates, while exhibiting excellentdielectric properties that allow monolithic integration, also typicallyexhibits high and inconsistent thermal expansion coefficient compared tothe metals and semiconductors of the CIGS layer stack. Thus, there existextrinsic stresses that combine with intrinsic stresses that can warp,wrinkle, distort and otherwise diminish the integrity of these flexiblesubstrates. In addition, the electrical and mechanical properties of aback contact also affect the device performance and adhesion.

FIG. 1 contains a graph of intrinsic stress state of sputtered Mo as afunction of Argon pressure during a vacuum-based sputtering Modeposition process. A careful balance of intrinsic and extrinsicstresses in the back contact deposition step is thus desirable toprovide a viable flexible photovoltaic device. The method of deposition,deposition pressure, rates, web process gasses, web speed, and number ofpasses are all variables that are balanced to provide the best backcontact for the device.

According to the present disclosure, a multilayer approach using two ormore different metals in the back contact is used to replace the priorMo film deposited onto both sides of a high-temperature polymericsubstrate. According to the disclosure, the polymeric substrate can be,for example, polyimide, polybenzobisoxazole (PBO), insulated metalfoils, or other such material for flexible, monolithically integratedCIGS modules using a high-temperature CIGS deposition process, such asmulti-source evaporation. Unlike prior processes which use Mo films onboth sides of the polymer in order to balance the stresses of thisprocess, along with subsequent CIGS, CdS and TCO depositions, accordingto some exemplary embodiments, a stress-balanced back contact is formedusing a dielectric film on the back side of the polymer substrate, aprimary high-conductivity but low-modulus and low-cost metallic filmlayer, for example, aluminum (Al), applied to the front side of thepolymer, followed by a thin cap of Mo over the Al film layer. The Mo maybe disposed onto the Al with or without added oxygen.

FIG. 2 contains a schematic cross-sectional view of a back contact for aflexible monolithically integrated CIGS photovoltaic device on a polymerutilizing a metallic multilayer as a top contact and an oxide as aback-side coating, according to some exemplary embodiments. Referring toFIG. 2, the polymer substrate 14 may be prepared to receive the disposedmaterials by plasma cleaning, annealing, or other processes best suitedfor a given combination of substrate and photovoltaic (PV) device. Theplasma treatment involves one or more gases. The amounts and percentageof each gas may vary to optimize the treatment for a particular materialbeing deposited. The power density of the plasma and the duration oftreatment may also be varied to optimize the treatment. Annealing orheating the substrate before, during, or after plasma treatment mayfurther optimize the treatment. The device 10 according to someexemplary embodiments includes the dielectric film 12, which can be, forexample, an oxide such as SiO₂, Al₂O₃, a nitride, an oxynitride such asan oxynitride of Al or Si, and which, in this particular exemplaryembodiment, is Al₂O₃, formed at the back side of the polymer substrate14. Other dielectric coating possibilities include high-temperaturesilicone, silicone resins, and other polyimides that may not have thestructural properties to function as a stand-alone substrate, but thathave high-temperature and high-emissivity properties and that arecapable of adding compressive stress to the polymer substrate. Anoptional adhesion layer 13 may be formed on the back side of the polymersubstrate 14 before the dielectric film 12 is formed. The adhesion layer13 can include at least one of molybdenum, aluminum, chromium, titanium,titanium nitride (TiN), a metal oxide, and a metal nitride. The optionaladhesion layer 13 can be made very thin, i.e., thin enough to have verylow conductivity and having little to no impact on the back sideemissivity. The optional adhesion layer 13 may oxidize some duringsubsequent oxide deposition of the dielectric film 12, forming, forexample, Mo oxide, Cr oxide, Ti oxide, etc. The polymer substrate 14 canbe, for example, polyimide, polybenzobisoxazole (PBO), insulated metalfoil, or other such material. Another optional adhesion layer 15 can beformed over the polymer substrate 14 to aid in adhesion of thesubsequent metallic film layer 16. The adhesion layer 15 can include atleast one of molybdenum, aluminum, chromium, titanium, titanium nitride(TiN), a metal oxide, and a metal nitride. The metallic film 16 isformed on the front side of the polymer substrate 14 or formed on thefront side of the adhesion layer 15 if it is present. The metallic film16 can be a high-conductivity but low-modulus and low-cost metallic filmmade of, for example, aluminum, copper, brass, bronze, or other suchmaterial. The thin cap layer 18 of Mo is formed over the metallic film16. The Mo cap layer 18 may be formed with or without added oxygen. TheCIGS layer 20 is formed over the Mo cap layer 18, which enables theproper chemical, mechanical and electrical interface to the CIGS layer20. A buffer layer 22, formed of, for example, CdS, may be formed overthe CIGS layer 20, and a transparent conductive oxide (TCO) layer 24 maybe formed over the buffer layer 22.

FIG. 3 contains an image of the dielectric-polymer-metal-Mo-CIGS stackstructure of the inventive concept, with various (four) thicknesses ofthe Al₂O₃ back side dielectric layer 12. The four exemplary thicknessesof the dielectric layer 12 are 0.0 nm (no back side dielectric layer orcoating), 210 nm, 350 nm and 640 nm. As illustrated in FIG. 3, accordingto the inventive concept, stress balancing is achieved. The combinationof back side dielectric film 12, the top-side metallic contacts 16 thatserve as the electrical back contact, and subsequent depositions, allbalance their respective stresses to achieve a flat material that isbetter suited for mass production processes.

Referring to FIG. 3, the stack of dielectric-polymer-metal-Mo-CIGSaccording to the inventive concept has very little compressive stresscompared to similar Mo-only back contact films. This is due to thepresence of the metal film 16. With the addition of the dielectric film12 on the back side, the substrate begins to flatten and at a thicknessof, for example, 640 nm, all stresses are balanced. According to someexemplary embodiments, depositing a film that can maintain sufficientelectrical conductivity while surviving a high-temperature CIGSdeposition process in which it is subjected to high temperatures(exceeding 400° C.) in a selenium (Se)-rich environment is a majoradvancement in the scale-up of flexible monolithically integrated CIGSdevices.

Mo presents a challenge in that, not only can the material exhibitdramatically different inherent stresses due to variations in processparameters, but mismatches in the coefficient of thermal expansion (CTE)between Mo and the underlying substrate coupled with high-temperatureprocessing, the stiffness of the substrate, and ultimately, themechanical properties of the subsequent films, can all lead to largestresses in the resultant multilayer construction. Mo can be depositedin various intrinsic stress states ranging from tensile to compressivein nature, as shown in FIG. 1. With as-deposited Mo films, a transitionbetween tensile and compressive intrinsic stresses in Mo occursapproximately at 6 mTorr with the compressive stress state exhibiting amaxima at approximately 1.2 Pa. However, regardless of the as-depositedstress state of Mo on the polymer, a compressive stress state is theresult of Mo on polymer after a high-temperature exposure, e.g., CIGSdeposition temperature. These stresses can lead to cracking of the thinfilms, or even the substrate, particularly if extrinsic stresses areadded in the form of bending or otherwise flexing the coated substrate.Stress balancing of the highly compressive Mo back contact, inconsideration of subsequent deposition steps, is achieved by depositinga compressive film to the substrate backside. In order to achieve a flatmaterial, the stress state is balanced, and as the top surface hasmultiple metal, semiconductor, and oxide layers, a corresponding Molayer applied to the bottom side of the substrate is required to balancethe multiple layers on the top side, although in most cases the type ofMo film used on the back side (for stress balancing) is depositeddifferently and to a different thickness than the Mo film on the front(for back side electrical conductor). Wrinkle reduction is one of theprimary reasons that batch processing of panels through the patterningcell is performed to prevent damage to the closely-moving ink headprinting operations. However balancing the front and back stresses ismuch more difficult when the stress levels are high.

Table 1 illustrates the challenge in depositing a metal, particularlyMo, onto a high-temperature polymeric substrate. Both Mo and Al have amuch higher modulus by an order of magnitude than the polymer, while thethermal expansion may be a closer match between Al and the polymer thanMo. More importantly, the yield stress of the Al is much lower than Mo,and the stress at 5% elongation of the polymer is closer to Al than Mo.Finally, the ultimate stress of the Mo is nearly twice that of thepolymer.

TABLE 1 MECHANICAL PROPERTIES OF ALUMINUM AND MOLYBDENUM COMPARED TO ATYPICAL HIGH-TEMPERATURE POLYMERIC SUBSTRATE Thermal Linear Young'sConductivity Expansion Melting Yield Ultimate Specific Modulus Poisson's(at 0° C.) Coefficient Point Stress Stress Metal Gravity GPa Ratio W/(m°K) ×10⁻⁶° C. K MPa MPa Al 2.7 68.95 0.33 237 25 933  30-140  60-140 Mo10.2 275.80 0.32 138 5 2893 585-690 690-827 Thermal Linear Stress atTensile Conductivity Expansion Max 5% Tensile Specific ModulusElongation (at 0° C.) Coefficient Temp Elongation Strength SubstrateGravity GPa (%) W/(m° K) ×10⁻⁶/° C. K MPa MPa UpilexR 1.5 6.9-9.1 42-500.29 12-20 ~723 210-260 360-520

In accordance with some exemplary embodiments, the overall stress statein the polymer is reduced, and, as a result, a more planar, wrinkle-freesubstrate is provided. Because Mo is used for a proper interface toCIGS, but is a major reason for the high stresses in the substrate,according to the inventive concept, its use has been minimized to theminimum required to mask the work function of the underlying primarymetallic film, as shown in Table 2. In some exemplary embodiments, theprimary metallic film of choice is aluminum (Al), although formulationsusing copper (Cu) and other highly electrically conductive materials,for example, brass or bronze, can be used. The CIGS device relies on theproper work function of its metallic back contact to function properly.While it is possible to use metallic foils (without insulting layers)with subsequent Mo deposition to mask the work function of the metalfoil substrate, the inherent stiffness of the non-polymeric substratesenables the ability to apply greater Mo film thicknesses without the Mostress overwhelming the substrate. With the polymeric process accordingto embodiments of the inventive concept, and their lower mechanicalproperties, the desirable masking effect by the Mo of the work functionof the underlying primary thin film back contact material (Al, Mo, etc.)is carefully balanced with the high stresses in Mo that can increasewith greater Mo thickness. Furthermore, the use of metallic foilswithout insulating layers precludes the straightforward ability tointegrate monolithically the photovoltaic device, and as such, limitdevice construction to discrete individual cells.

TABLE 2 ELECTRICAL PROPERTIES OF ALUMINUM AND MOLYBDENUM (AT 20° C.)COMPARED TO A TYPICAL POLYMERIC SUBSTRATE Material ρ [Ω m] σ [S/m] WorkFunction (eV) Aluminum 2.82 × 10⁻⁸ 3.5 × 10⁷ 4.08 Molybdenum 5.34 × 10⁻⁸1.8 × 10⁷ 4.60 Upilex PI ~10⁺¹⁷ ~10⁻¹⁷ Ń

The AlMo stack of some exemplary embodiments provides several advantagesover conventional single or multi-layer Mo back contacts.

-   -   1) The film can be made with the bulk of the stress state        dictated by the Al film 16, which is far thicker than the Mo cap        18. Thus, the overall stress state in the front side        metallization is reduced.    -   2) The AlMo stack achieves a far greater electrical in-plane        conductivity than the baseline Mo film, exceeding an order of        magnitude improvement as is shown in Table 2. This results in        the ability to carry greater current than prior devices, and        enables greater cell pitch (width) for monolithically integrated        modules. Larger cells equates to fewer interconnects, which        reduces the interconnect-related losses. Measurements with        samples indicate an order of magnitude reduction in sheet        resistance, dropping from baseline 2 Ω/square to 0.2 Ω/square.        This improvement allows for cell width (pitch) to increase to        almost double that demonstrated in baseline conditions, thereby        reducing the interconnects by a factor of two as well.    -   3) While Mo has adequate electrical conductivity for some        applications, it constrains the performance of CIGS that        possesses high current density (>30 mA/cm²). By using only a        thin Mo cap 18, and relying on the conductivity of Al to provide        the bulk of the electrical conductivity, the stacked material of        the embodiments provides very little sheet resistance. Table 2        also compares the electrical properties of Cu, Al and Mo. Mo has        approximately half the electrical conductivity of Al and less        than ⅓ the electrical conductivity of Cu. However, as the work        function of Al is significantly lower than that of Mo, and that        Al would diffuse readily into CIGS, a cap of Mo is retained to        shield the low Al work function from CIGS. Likewise, Cu would        diffuse into the CIGS during deposition when using Cu, brass or        bronze as metal layer 16. Thus, by using a Mo cap, the best        electrical properties are retained while providing the proper        work function interface to ensure a successful photoelectric        effect.    -   4) As an added benefit to the electrical conductor construction,        the thin Mo cap 18 presents a much lower electrical resistance        pathway through the P2 laser scribe, e.g., via scribe, into the        higher conductivity Al. Thus, while the baseline P2 interconnect        resistance under the process of record (POR) is nominally        between 500-1000 mΩ-cm, the P2 resistance for this new        interconnect drops to 2 mΩ-cm. This alone will account for        approximately 5% boost in power output for a given module by        reducing module losses.

Because Mo that is sufficiently thick to provide adequate electricalconductivity on polymer contributes adversely to the stress state in thephotovoltaic stack, minimizing the Mo content of the device back contactallows for another material, other than Mo, to serve as a back-sidefilm, according to exemplary embodiments. According to the presentdisclosure, by eliminating dependence upon Mo on the back-side film, andby minimizing it in the back contact, significant advantages over theprior art are realized.

-   -   a) Mo serves as an interface to the CIGS, and thus, masks the Al        work function in order to allow the device to work optimally.        Other metallic elements or alloys can be utilized as desired for        new substrates as they become available.    -   b) The overall reduced stress state in the back contact film        provides options for the back side film. In one case, an        inexpensive alumina (Al₂O₃) film that is a good insulator and        provides some level of moisture protection for the polymer can        be employed. However, other oxide films can be employed to        enhance bonding strength to packaging, and oxynitrides can be        substituted for better moisture protection as well.    -   c) By virtue of reducing the stress state in the films on either        side of the polymer substrate, the resultant stress experienced        by the polymer is also reduced. Particularly for        high-temperature polymers used in roll-to-roll deposition, the        reduced stress state will result in reduced wrinkling and        waviness of the web, particularly after high-temperature        excursions such as those experienced in CIGS deposition.    -   d) As new flexible, non-conductive substrates are developed,        such as Poly (p-phenylene-2,6-benzobisoxazole) (PBO) and are        introduced into the flexible CIGS market, experience in reducing        the stress imparted by the back contact can result in a        construction that may eliminate the need for the back-side film        altogether.    -   e) As achieving the desired Mo stress state is important,        deposition rate is limited with standard Mo films, often        requiring multiple thinner passes to achieve the desired        electrical and stress properties. State-of-the-art films using        the process of record (POR) is 390 nm on the front side and 620        nm on the back side of the substrate, for a total of just over a        micron (1,010 nm). The nominal Mo thickness with the new        construction according to the exemplary embodiments is        approximately 100-200 nm, or an 80-90% reduction in the amount        of Mo in the device. Using the back contact of the exemplary        embodiments significantly reduces the need to deposit in        multiple layers, and furthermore, as the film is significantly        thinner, at least a 5× throughput increase from the back contact        chambers should result, as Al is much easier to deposit at high        web rates.    -   f) Mo is a relatively expensive film in the CIGS device, and is        approximately 35 times the cost of Al. As noted above, Mo        reduction and substitution of common elements (Al, Al₂O₃)        reduces the cost of the back contact dramatically. Even in        replacing the back-side Mo with Al₂O₃ should have a noticeable        effect.    -   As noted above, the Al—Mo back contact has demonstrated        dramatically lower sheet resistance and P2 interconnect        resistance. Combined, these effects will account for a        percentage point of efficiency when module design is optimized        to take full advantage of the effects. Even with the same module        design, module power should increase by 5% due to the reduced P2        resistance.

According to the exemplary embodiments, elimination of a metal back sidefilm and replacing it with a dielectric layer provides thermalmanagement in the device, in addition to stress management, as describedherein in detail. Heating of substrates in a vacuum includes conductiveheating (direct contact to a substrate) and/or radiative heating (energyradiating from one source to another). Radiative heating is the mostcommon means of transferring thermal energy to the substrate, but thedegree to which energy is conveyed is dependent upon the substrate'sabsorptivity (ability to absorb energy) and emissivity (ability toradiate heat into the environment). Metals typically have loweremissivity than, for example, oxide films; thus, metal surfaces do notgive up their heat as easily as oxides. Thus a polymer coated with metalon both sides can trap the heat within the sandwiched polymer substrate.In a vacuum, a surface coated with a high-emittance coating, such as anoxide or nitride, can provide radiative cooling to that surface and thesubstrate. A cooler back side coating and substrate helps to keep thesubstrate from degrading and embrittling during high device-sidetemperatures, and thus enables higher device-side temperatures that canlead to higher quality solar absorber layers.

Applicant has additionally determined that it is desirable that Mo caplayer 18 be relatively dense to minimize diffusion of metal, such asaluminum or copper, from metallic film layer 16 into CIGS layer 20. Forexample, in some embodiments, Mo cap layer 18 has a density of at least85% of the bulk density of molybdenum so that Mo cap layer 18 acts as adiffusion barrier, thereby potentially enabling aluminum, copper, orother metal, of metallic film layer 16, to be disposed adjacent to Mocap layer 18 without significant diffusion of the metal through Mo caplayer 18. High density of Mo cap layer 18 is obtained, for example, byusing a low-pressure vacuum-based sputter deposition process to depositMo cap layer 18. For example, in a particular embodiment, Mo cap layer18 is deposited by a vacuum-based sputter deposition process at apressure of less than 20 millitorr (mTorr), preferably at less than 6mTorr, to obtain high density of Mo cap layer 18.

In some embodiments, Mo cap layer 18 includes a plurality of sublayers,where a sublayer closest to metallic film layer 16 has a high density,and one or more other sublayers further from metallic film layer 16 havelower densities. For example, FIG. 4 is a schematic cross-sectional viewof a device 400, which is similar to device 10 of FIG. 2, but where Mocap layer 18 is replaced with a bilayer Mo cap layer 418. Mo cap layer418 includes a first sublayer 426 disposed on metallic film layer 16 anda second sublayer 428 disposed on first sublayer 426. First sublayer 426has a high density and therefore acts as a diffusion barrier to preventdiffusion of metal from metallic film layer 16 into CIGS layer 20.Second sublayer 428, on the other hand, has a lower density than firstsublayer 426 and therefore does not substantially inhibit diffusion. Ina particular embodiment, first sublayer 426 is deposited by avacuum-based sputter deposition process at a pressure of less than 20mTorr, preferably at less than 6 mTorr, to obtain high density, andsecond sublayer 428 is deposited by a vacuum-based sputter depositionprocess at a pressure greater than that used to deposit first sublayer426, such that second sublayer 428 has a lower density than firstsublayer 426.

Moreover, Applicant has additionally determined that it may be desirableto deposit dielectric layer 12 before metal film layer 16 and Mo caplayer 18 (or bilayer Mo cap layer 418) in embodiments including optionaladhesion layer 13. In particular, adhesion layer 13 is typically atleast slightly electrically conductive, and presence of adhesion layer13 may therefore cause arcing if metallic film layer 16 and/or Mo caplayer 18 are deposited by a sputter process. Deposition of dielectriclayer 12, however, insulates adhesion layer 13. Thus, deposition ofdielectric layer 12 before depositing metallic film layer 16 and Mo caplayer 18 reduces the likelihood of arcing during sputter deposition ofmetallic film layer 16 and Mo cap layer 18.

FIG. 5 illustrates a method 500 for forming a photovoltaic element. Instep 502, a first adhesion layer is disposed on a back side of a polymersubstrate. In one example of step 502, adhesion layer 13 is disposed onthe back side of polymer substrate 14 (FIG. 2). In step 504, adielectric layer is disposed on the adhesion layer. In one example ofstep 504, dielectric layer 12 is disposed on adhesion layer 13. In step506, a metal structure is disposed on a device side of the polymersubstrate, after the step of disposing the dielectric layer, where thedevice side is opposite of the back side. In one example of step 506,metallic film layer 16 and Mo cap layer 18 are disposed on the deviceside of substrate 14, after adhesion layer 13 and dielectric layer 12are disposed on the back side of substrate 14. In step 508, a CIGSphotovoltaic structure is disposed on the metal structure. In oneexample of step 508, CIGS layer 20 is disposed on Mo cap layer 18.

FIG. 6 illustrates another method 600 for forming a photovoltaicelement. In step 602, a dielectric layer is disposed on a back side of apolymer substrate. In one example of step 602, dielectric layer 12 isdisposed on the back side of polymer substrate 14 (FIG. 2). In step 604,a metallic film layer is disposed on a device side of the polymersubstrate, where the device side is opposite of the back side. In oneexample of step 604, metallic film layer 16 is disposed on the deviceside of substrate 14. In step 606, a molybdenum cap layer is disposed onthe metallic film layer using a vacuum-based sputter deposition processat a pressure of less than 20 mTorr. In one example of step 606, Mo caplayer 18 is disposed on metallic film layer 16 using a vacuum-basedsputter deposition process at a pressure of less than 20 mTorr. In step608, a CIGS photovoltaic structure is disposed on the molybdenum caplayer. In one example of step 608, CIGS layer 20 is disposed on Mo caplayer 18.

While it is desirable for the backside layer to be dielectric, in somealternate embodiments, dielectric layer 12 is replaced with a backsidemetal layer, where the backside metal layer balances stress resultingfrom layers on the device side of polymer substrate 14. For example,FIG. 7 is a schematic cross-sectional view of a device 700, which issimilar to device 10 of FIG. 2, but where dielectric layer 12 isreplaced with a backside Mo layer 712. In certain of these embodiments,backside Mo layer 712 is deposited by a vacuum-based sputter depositionprocess at a pressure of less than 6 mTorr, preferably at less than 3mTorr. Applicants have discovered that these sputter depositionconditions are particularly effective in balancing stress from layers onthe device side of polymer substrate 14. Further stress matching canpotentially be achieved by tuning the thickness of backside Mo layer712. Additionally, in some embodiments, backside Mo layer 712 is about10 percent to 30 percent oxygen in the ambient gas during the depositionprocess, whereas the presence of oxygen can also modify the stress.Backside Mo layer 712 could be formed of a material other than Mowithout departing from the scope hereof.

Exemplary embodiments have been described herein. For example, exemplaryembodiments have been described in terms of specific exemplary polymericsubstrates and particular exemplary coatings or layers. It will beunderstood that the exemplary embodiments relate to stress balancing toprovide a back contact in an improved photovoltaic device. Therefore,the present disclosure is applicable to other substrate materials andother back side coatings or layers. In fact, the disclosure may beapplicable to structures with alternate substrates and the eliminationof back side coating altogether.

Combinations of Features

Various features of the present disclosure have been described above indetail. The disclosure covers any and all combinations of any number ofthe features described herein, unless the description specificallyexcludes a combination of features. The following examples illustratesome of the combinations of features contemplated and disclosed hereinin accordance with this disclosure.

In any of the embodiments described in detail and/or claimed herein, thephotovoltaic element can comprise a CIGS structure.

In any of the embodiments described in detail and/or claimed herein, thedielectric can comprise at least one of SiO₂, Al₂O₃, and silicone resin.

In any of the embodiments described in detail and/or claimed herein, athin adhesion layer can be disposed between the layer of dielectric andthe back side of the polymer substrate.

In any of the embodiments described in detail and/or claimed herein, theadhesion layer can comprise at least one of Mo, Cr, and Ti.

In any of the embodiments described in detail and/or claimed herein, themetal structure can comprises a first metal layer, the first metal layercomprising at least one of aluminum, brass, bronze and copper. The metalstructure is optionally disposed on the polymer substrate after thedielectric layer is disposed on the polymer substrate.

In any of the embodiments described in detail and/or claimed herein, themetal structure can further comprise a layer of molybdenum formed overthe first metal layer. The layer of molybdenum optionally has a densityof at least 85% of the bulk density of molybdenum. The layer ofmolybdenum is optionally formed at least partially using a vacuum-basedsputter deposition process at a pressure of less than 20 mTorr. Thelayer of molybdenum optionally includes a plurality of sublayers, wherea sublayer closest to the first metal layer is formed using avacuum-based sputter deposition process at a pressure of less than 20mTorr, and one or more sublayers further from the first metal layer areformed using a vacuum-based sputter deposition process at a pressure ofgreater than that used to form the sublayer closest to the first metallayer.

In any of the embodiments described in detail and/or claimed herein, themetal structure can further comprise a thin adhesion layer disposedbetween the first metal layer and the device side of the polymersubstrate.

In any of the embodiments described in detail and/or claimed herein, thethin adhesion layer can comprise at least one of molybdenum, aluminum,titanium and chromium.

In any of the embodiments described in detail and/or claimed herein, themetal structure can further comprise a thin adhesion layer in contactwith the device side of the polymer layer.

In any of the embodiments described in detail and/or claimed herein, thethin adhesion layer can comprise at least one of molybdenum, aluminum,chromium, titanium nitride (TiN), a metal oxide, and a metal nitride.

In any of the embodiments described in detail and/or claimed herein, thedielectric layer may be replaced with a backside metal layer formedusing a vacuum-based sputter deposition processes at a pressure of lessthan 6 mTorr. The backside metal layer can be formed of a Mo layer,where the Mo layer is optionally 10% to 30% oxygen in the ambient gasduring the deposition process.

While the present disclosure makes reference to exemplary embodiments,it will be understood by those of ordinary skill in the art that variouschanges in form and details may be made therein without departing fromthe spirit and scope of the present disclosure.

What is claimed is:
 1. A method for forming a photovoltaic element,comprising: disposing a dielectric layer over a back side of a polymersubstrate; after the step of disposing the dielectric layer, disposing ametal structure on a device side of the polymer substrate, the deviceside being opposite of the back side; and disposing a CIGS photovoltaicstructure on the metal structure, the step of disposing the metalstructure including: disposing a metallic film layer on the device sideof the polymer substrate, and disposing a metallic cap layer on themetallic film layer, the metallic cap layer having a differentcomposition than the metallic film layer.
 2. The method of claim 1, themetallic cap layer comprising molybdenum.
 3. The method of claim 2, themetallic film layer comprising at least one of aluminum, brass, bronze,and copper.
 4. The method of claim 3, the step of disposing the metalliccap layer comprising disposing the metallic cap layer using avacuum-based sputter deposition process at a pressure of less than 20millitorr.
 5. The method of claim 3, wherein: the metallic cap layercomprises first and second sublayers each formed of molybdenum, thefirst sublayer disposed on the metallic film layer, the second sublayerdisposed on the first sublayer, and the CIGS photovoltaic structuredisposed on the second sublayer; and the step of disposing the metalliccap layer comprises: (a) disposing the first sublayer using avacuum-based sputter deposition process at a first pressure of less than20 millitorr, and (b) disposing the second sublayer using a vacuum-basedsputter deposition process at a second pressure, the second pressurebeing greater than the first pressure.
 6. The method of claim 3, furthercomprising, before the steps of disposing, annealing the polymersubstrate.
 7. The method of claim 3, further comprising, before thesteps of disposing, plasma cleaning the polymer substrate.
 8. The methodof claim 7, further comprising, before the steps of disposing, annealingthe polymer substrate.
 9. The method of claim 1, the dielectric layercomprising at least one of SiO₂, Al₂O₃, and silicone resin.
 10. Themethod of claim 1, further comprising disposing a second adhesion layeron the device side of the polymer substrate, before the step ofdisposing the metal structure on the device side of the polymersubstrate, the second adhesion layer comprising at least one ofmolybdenum, aluminum, chromium, titanium, titanium nitride (TiN), ametal oxide, and a metal nitride.
 11. The method of claim 1, furthercomprising disposing a first adhesion layer on the back side of thepolymer substrate, before the step of disposing the dielectric layerover the back side of the polymer substrate.
 12. The method of claim 11,the first adhesion layer comprising at least one of molybdenum,aluminum, chromium, titanium, titanium nitride (TiN), a metal oxide, anda metal nitride.
 13. A method for forming a photovoltaic element,comprising: disposing a dielectric layer on a back side of a polymersubstrate; disposing a metallic film layer on a device side of thepolymer substrate, the device side being opposite of the back side;disposing a molybdenum cap layer on the metallic film layer at leastpartially using a vacuum-based sputter deposition process at a pressureof less than 20 millitorr; and disposing a CIGS photovoltaic structureon the molybdenum cap layer.
 14. The method of claim 13, the metallicfilm layer comprising at least one of aluminum, brass, bronze, andcopper.
 15. The method of claim 14, further comprising, before the stepsof disposing, annealing the polymer substrate.
 16. The method of claim14, further comprising, before the steps of disposing, plasma cleaningthe polymer substrate.
 17. The method of claim 16, further comprising,before the steps of disposing, annealing the polymer substrate.
 18. Themethod of claim 14, the dielectric layer comprising at least one ofSiO₂, Al₂O₃, and silicone resin.
 19. The method of claim 13, themolybdenum cap layer comprising first and second molybdenum sublayers,the step of disposing the molybdenum cap layer on the metallic filmlayer comprising: disposing the first molybdenum sublayer on themetallic film layer using a vacuum-based sputter deposition process at afirst pressure of less than 20 millitorr; and disposing the secondmolybdenum sublayer on the first molybdenum sublayer using avacuum-based sputter deposition process at a second pressure, the secondpressure being greater than the first pressure.
 20. A method for forminga photovoltaic element, comprising: disposing a backside metal layer ona back side of a polymer substrate using a vacuum-based sputterdeposition process at a pressure of less than 6 millitorr; disposing ametallic film layer on a device side of the polymer substrate, thedevice side being opposite of the back side; disposing a molybdenum caplayer on the metallic film layer; and disposing a CIGS photovoltaicstructure on the molybdenum cap layer.
 21. The method of claim 20, thebackside metal layer comprising molybdenum.
 22. The method of claim 20,the metallic film layer comprising at least one of aluminum, brass,bronze, and copper.
 23. The method of claim 22, wherein the step ofdisposing the molybdenum cap layer comprises using a vacuum-basedsputter deposition process at a pressure of less than 20 millitorr. 24.The method of claim 22, the molybdenum cap layer comprising first andsecond molybdenum sublayers, the step of disposing the molybdenum caplayer on the metallic film layer comprising: disposing the firstmolybdenum sublayer on the metallic film layer using a vacuum-basedsputter deposition process at a first pressure of less than 20millitorr; and disposing the second molybdenum sublayer on the firstmolybdenum sublayer using a vacuum-based sputter deposition process at asecond pressure, the second pressure being greater than the firstpressure.
 25. The method of claim 22, further comprising, before thesteps of disposing, annealing the polymer substrate.
 26. The method ofclaim 22, further comprising, before the steps of disposing, plasmacleaning the polymer substrate.
 27. The method of claim 26, furthercomprising, before the steps of disposing, annealing the polymersubstrate.